1. Field of the Invention
This invention is in the field of thermoplastic hot melt compositions which have superior physical properties for casting, encapsulation, extrusion, or injection molding and potting.
2. DESCRIPTION OF THE PRIOR ART
Thermoplastic hot melt compositions are commercially used in many applications such as in adhesive bonding, decorative and protective coating, casting, encapsulation, laminating and molding. Such hot melt compositions may include products containing only a single ingredient such as a wax, for example, beeswax, a resin such as rosin or a polymer such as polystyrene or mixtures comprising many combinations of organic materials.
Hot melt compositions have also been used for fixturing matrices and as pattern materials for making molds used for investment casting. When used in these two fields, hot melts require very specific combinations of properties which make them quite distinct from other types of hot melts. Generally, they must have very low melt viscosities at relatively low temperatures nearly coinciding with their softening range.
Heretofore, much of the prior art of matrix fixturing revolves around the use of low melting metal alloys having controlled shrinkage characteristics. These low melting alloys provide a matrix for holding a wide range of metal parts for drilling, grinding, or lapping, milling and turning operations. Certain metal alloys in which bismuth or antimony are incorporated, provide molten metal alloys which expand upon solidification thus providing excellent duplication and reproduction. The use of such alloys is, however, limited in some areas. The weight of these alloys in fixturing large parts poses a processing problem and their high cost provides an expensive inventory problem. Consequently, the loss of any of the alloy during processing is an economic disadvantage. Furthermore, contamination caused by bismuth, cadmium, lead and tin contained in such alloys is extremely damaging in the fixturing of jet engine parts particularly those composed of expensive superalloy materials.
In order for a matrix to function effectively as a fixturing material for jet engine blade work, it has generally been accepted that the matrix material must have certain physical characteristics. For one, the material should have a rigidity approximating that of lead. It should be pourable and have a setup time of about 2 minutes. The shrinkage and cavitation characteristics should be at a minimum. The matrix material must be dimensionally stable for a period of 6 to 8 weeks and provide good adhesion to the blade and matrix box. It must be relatively nontoxic, inexpensive and reusable. It should have a shelf life of over 3 months. It must evidence good cohesion properties when sequentially cast. It must be readily removable without damaging the parts. It should not be appreciably soluble in machining coolants such as high sulfur base oils at temperatures of 140.degree. F. Furthermore, the fixturing material or its residue should not be corrosive to parts or coatings of the jet engine blade up to temperatures of 2000.degree. F.
Equally stringent requirements are present in compositions useful as pattern materials for producing investment casting molds. As is well known in the art, the mold making process involves the use of a disposable pattern which is usually injection molded in a pattern die. After the pattern is formed, it is sequentially coated with successive layers of refractory particles, alternating between layers of very finely divided particles and relatively coarse or stucco layers. After a sufficient number of layers has been built up, the disposable pattern is removed by firing or by steam autoclaving or similar means to leave a relatively rigid but still fairly permeable investment casting mold having a smooth surface.
The selection of a suitable pattern material is of substantial importance in the overall process of investment mold making. Among the many properties which must be considered in any pattern material are:
(1) its ash content, PA1 (2) the production of a suitable surface finish, PA1 (3) resistance to the primary coat, binder and carriers used in the investment slurries, PA1 (4) resistance to gum formation in the injection machine, PA1 (5) low viscosity, at low melt temperatures, PA1 (6) suitable strength, both tensile and impact, PA1 (7) a suitable ductile-to-ductile transition temperature range, PA1 (8) a suitable solidification temperature range, PA1 (9) oxidation resistance, PA1 (10) wettability, PA1 (11) its weld strength or adhesiveness so that patterns may be joined together and the pattern may be joined to a suitable sprue, PA1 (12) suitable coefficients of thermal expansion, shrinkage, dish, cavitation and dish, cavitation and sink, PA1 (13) suitable hardness, PA1 (14) suitable softening temperature, and PA1 (15) lack of toxicity. PA1 Fatty acids 20-100 parts by weight PA1 Formaldehyde (100%) 10-30 parts by weight PA1 Urea 20-40 parts by weight PA1 50-150 parts of a hydrocarbon resin (e.g., cross-linked polyolefinic) PA1 5-20 parts of a flexible polymer PA1 10-40 parts wax PA1 50-350 parts inorganic filler PA1 1-15 parts cellulosic derivative (e.g., ethyl cellulose) PA1 1-40 parts wax
In addition to the foregoing physical characteristics, for purposes of economy, the pattern material should be reclaimable by ordinary recovering procedures, and must be composed of relatively inexpensive, commercially available materials.